Cloud releases: Switch to firstboot_pkg_upgrade

Cloud images are deployed with base system packages. Introduce a
firstboot package auto updater to patch the base system on first boot.

Approved by: re (cperciva)
MFC after: 1 hour
MFC to: stable/15
Reviewed by: cperciva
Sponsored by: Google Cloud
Differential Revision: https://reviews.freebsd.org/D56890

(cherry picked from commit 464a351267dc0d1843b919dd72ad1c70c24815ce)
(cherry picked from commit 0bb2b2a45f3c0c147d7c55e010be45e55af8df87)